A buckling restrained brace (BRB) is typically used in buildings or other structures to brace them from earthquake or other lateral forces. They are placed diagonally in buildings and are seen as sloping diagonal members running from floor to floor, sometimes visible in the building windows. A BRB is a structural brace meant to resist compression, and designed to not buckle. All other braces will buckle, similarly to a drinking straw, if you push axially on the ends of it. A BRB separates the buckling behavior from the load carrying capacity. A simple experiment to demonstrate this behavior is to take a 20″ long ⅛″ diameter steel rod and compress it axially. Buckling of the rod will be seen with very little applied axial force (from the ends of the straws). Now take this same rod and place it through an 18″ long ½″ diameter steel pipe and apply an axial load and you will see it can now sustain orders of magnitude more force. The same experiment, on a less dramatic scale, could be done with a plastic straw and a section of ½″ PVC pipe. The rod, the “load carrying element” (LCE), can now sustain more load because of the pipe the “buckling restraining element” (BRE). The LCE and BRE perform two independent but complementary roles. The LCE takes the force/loading only. The BRE only has to prevent the buckling and does not sustain any load. The LCE and BRE behaviors are bifurcated. On the other hand, a typical brace must carry load and prevent buckling with the same element.
A BRB takes this concept even further. If one can control the environment between the LCE and the BRE precisely enough you can distort the LCE's molecular structure. The LCE can be smashed axially in compression and then stretched in tension over and over until the material finally reaches its ductility limits. This is the same phenomenon as when you bend a paper clip. You can bend it back and forth for a while, but if you keep going it reaches its limits and breaks. The BRB LCE is similar, except instead of bending, it's smashing and stretching. It is worth mentioning that the BRE is not needed when the LCE is in tension. In tension mode, buckling is impossible. Thus in tension, the BRE is just along for the ride and it is only necessary when the BRB is being smashed in compression. The ability of the BRB to smash and stretch over and over again with relatively large displacements makes it possible to absorb large amounts of earthquake or other lateral forces much like a shock absorber.
All of the current producers use similar art. They all take a long slender rod, the LCE, which is typically called the “steel core” or “core plate” and pass it through a hollow steel tube or pipe. Once the core plate is placed through the pipe/tube, the annular space between the core plate and the pipe is filled with a rigid cementitious material, like concrete. The pipe and the concrete are called the “casing”, which is the BRE. Thus, a BRB is basically a large steel rod (2″ diameter for instance) passed through a 12″ steel pipe that is centered in the pipe, with concrete filling the space between the rod and the pipe.
If the concrete were in intimate contact with the core plate, there would be no room for the core plate to expand as it is smashed from the ends. As the core plate expands it would press against the concrete, thus engaging the concrete and subsequently the pipe casing. This is the same reaction as a typical foam ear plug. If it is compressed from the two ends it gets fatter (thicker and shorter). The material has to move somewhere. The same thing happens to the core plate but not quite as dramatically. This is the crux of where the art between all the producers varies. You cannot just place the concrete up tight against the core plate. The main reason is because when the core plate smashes, the molecular structure must be relieved by expanding laterally. If the core expands and the concrete is tight, it will seize up against the concrete and transfer the load carrying duties to the concrete and pipe casing. Keep in mind that the concrete and pipe are only designed to prevent buckling and not to take any load. If those elements are also engaged in taking the load/force, they will tend to buckle. Thus great care must be taken such that the core has a zone of separation from the concrete, and the core plate is unbonded from the concrete, so it can move independently from the concrete, and can expand inside the concrete under compressive force. In other words, you need a small gap or layer of film between the core and the concrete to accommodate this behavior.
To further complicate this, if you leave too much gap between the core and the concrete, as the core smashes, it will try to buckle up against the concrete. This buckling behavior is denoted by a series of sinusoidal shaped waves. As the load on the core increases the number of equidistant waves also increases along the core plate length. This wave shaped core will impart transverse forces into the concrete and pipe that can degrade the concrete and cause the BRB to fail. Typically, if this behavior is not controlled, the concrete breaks out as well as the walls of the pipe or tube. The larger the gap between the core and the concrete the larger the amplitude of the buckling and the larger the transverse forces will be. Also, this behavior creates friction between the core and the concrete which decreases the quality of the performance by making its compressive capacity much larger than its tension capacity. This is undesirable in regulatory building codes because it causes the rest of the structure to be more robust and expensive than required. Thus the true art is how well you can control this environment between the core and the concrete, how economically you can do it and still achieve the highest performance standards. This is achieved by providing precise spacing around the core plate, neither too small nor too large, and unimpaired movement of the core plate inside the concrete, while utilizing minimal cost in materials and manufacturing. Doing such will provide the ability for the BRB to sustain repeated loads in multiple events most cost effectively.
One critical performance standard is the difference in what compressive force it takes to deform the BRB verses what force it takes to deform the BRB the same amount in tension. Remember that in tension the concrete and pipe are just along for the ride. But in compression the core tries to buckle up against the concrete, creating friction. Also remember that when the core smashes it swells (expands). This creates more area to smash which requires more force. In tension the core is not buckling against the concrete and it is shrinking, resulting in less resistance from contact with the concrete and less force required to stretch it. The manufacturers can't do anything about the swelling and shrinking of the core plate but they can reduce the friction against the concrete by controlling the amplitude of the equidistant sinusoidal buckling waves and by providing bearing materials between the core and the concrete. The closer the manufacturers can match the compressive and tension behaviors the lighter they can make the overall building structures. Thus creating a well controlled gap between the core and the concrete is essential for performance.
Another critical performance standard is how much the BRB can smash and stretch cumulatively. This is also improved by how well the gap is controlled between the core and the concrete. The smaller you can keep the amplitude of the sinusoidal buckling core or bending of it the more it can smash and stretch because less of its deformational capacity is used up in bending. But remember the gap cannot be too small or else the swelling of the core cannot be accommodated. Thus the gap needs to be optimized to allow for swelling of the core while keeping the amplitude of the buckling waves small.
Shridhara is an early patent in this technology. Shridhara's patent defines the interface between the core and the concrete as a “gap”. The patent does not reveal how the gap is controlled nor does it even say how to create it during manufacture.
Nippon (Unbonded Brace) uses a “film” (reports are that it is really “ice and water shield” type roofing product) with the film having a large variance in secant modulus (Ratio of stress to strain at any point on curve in a stress-strain diagram. It is the slope of a line from the origin to any point on a stress-strain curve) from that of almost petroleum jelly to concrete.
CoreBrace uses a bearing material Ultra High Molecular Weight (UHMW) polymer (the base material on snow skis) between the core and concrete that is separated from the core via separators that are then removed after the concrete is placed, creating a gap. They are fairly precise about the bearing material, spacers and gaps it creates. They also have numerous other patents in regard to the device, one of which the inventor of this technology is listed as a co-inventor.
Star Seismic uses a metal sheet between the concrete and the core and then removes the sheet after the concrete solidifies, creating a gap. They also have several other patents in regard to other elements of the BRB.
When the core plate compresses or stretches a little, like a rubber band, it will spring back to its original shape. This called “elastic” behavior, hence the term “elastic” bands. However, at large deformations, the core plates will permanently distort and will not rebound to its original shape, which is called “plastic” behavior. When steel goes into its “plastic” behavior and the molecular structure is permanently distorted. So in compression the steel molecules flatten and spread out. In tension they lengthen and get thinner. This plastic behavior is why the region between the core plate and the concrete is so critical. This plastic behavior is also what absorbs the large seismic forces. These forces literally smash and stretch the BRB plastically back and forth acting like a fuse for the seismic energy.